1. Field of the Invention
The present invention relates to a rechargeable battery. More particularly, the present invention relates to a rechargeable battery that includes a cap assembly.
2. Description of the Related Art
Unlike a primary battery, which cannot be recharged, a rechargeable battery can be repeatedly recharged and discharged. A low capacity rechargeable battery (e.g., a rechargeable battery composed of one cell) can be utilized as a power source for driving a small and portable electronic device, such as a mobile phone, a laptop computer and a camcorder. A large capacity rechargeable battery (e.g., a rechargeable battery composed of a plurality of cells connected to form a pack) can be utilized as a power source for driving a motor of a hybrid vehicle.
Rechargeable batteries are produced in various suitable shapes, and typically in a shape of a cylinder or polygon.
Further, a plurality of rechargeable batteries (or battery cells) can be serially connected and included as part of a high capacity rechargeable battery module such that the module can be used for driving a motor of an electric vehicle that needs a large amount of electrical power.
A rechargeable battery is composed of an electrode assembly having a positive electrode, a negative electrode, and a separator between the positive electrode and the negative electrode; a case containing the electrode assembly; and a cap assembly for closing an opening side of the case.
In the case of a cylindrical rechargeable battery, the positive electrode and the negative electrode include parts in the electrode assembly that are not coated with an active material. The positive electrode and negative electrode uncoated parts face opposite directions.
A negative current collector plate is attached to the negative uncoated part, and a positive current collector plate is attached to the positive uncoated part. The negative current collector plate is electrically connected to the case, and the positive electrode current collector plate is electrically connected to the cap assembly and is for supplying the current out of the electrode assembly. Therefore, the case operates as a negative terminal, and a cap-up disposed at the cap assembly operates as a positive terminal.
While a rechargeable battery is repeatedly recharged and discharged, a gas builds up inside the rechargeable battery and internal pressure rises. Thus, if unchecked, the increased internal pressure may cause an explosion of the rechargeable battery. In order to protect from (or prevent) the explosion, a vent plate is provided under the cap-up, having a notch, such that the vent plate fractures at a certain level of pressure.
The vent plate includes a convex portion facing downward, and the convex portion is welded to a sub-plate that is electrically connected to the electrode assembly. When the pressure rises inside the rechargeable battery, the convex portion is first disengaged from the sub-plate, interrupting electric current flow between the vent plate and the sub-plate. If the pressure further increases, the vent plate fractures at the notch, thereby discharging gas to the outside.
A current interrupt pressure (at which electric current is interrupted) and a vent fracture pressure (at which the notch at the vent plate) is fractured are important elements in the aspects of reliability and safety of the rechargeable battery.
That is, if the current interrupt pressure and the vent fracture pressure are set to be low, the safety of the battery increases but this leads to a problem that the battery does not operate even with a small increase in internal pressure.
Particularly, a rechargeable battery having a lithium(li)-nickel(ni)-cobalt(co)-manganese(Mn) oxide(O2) (hereinafter “NCM”)-based positive active material, shows a different pressure-behavior than a rechargeable battery with a lithium cobalt oxide (LiCoO2, hereinafter “LCO”)-based positive active material. As such, the rechargeable battery with an NCM-based positive active material should have different current interrupt pressure and vent fracture pressure than those of a rechargeable battery with an LCO-based positive active material.
If the current interrupt pressure and vent fracture pressure derived for an LCO-based positive active material are directly applied to a battery with an NCM-based positive active material, there will be a problem of reduced reliability and safety of the rechargeable battery.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.